net: tcp: Fix IPv6 TCP MSS issue

Fixes #56657, incorrect TCP MSS calculation for IPv6.

Previously the supported MSS would incorrectly returned as the MTU if it
was less than the MTU, probably to account for the case where
the network interface MTU check returns 0.

New behaviour is to return the supported MSS as MTU minus header length,
using default MTU for this calculation if network interface MTU check
returns 0.

Signed-off-by: Kenny Johansson <wirehell@gmail.com>